IL108407A

IL108407A - Cutting insert

Info

Publication number: IL108407A
Application number: IL10840794A
Authority: IL
Original assignee: Sandvik Ab
Priority date: 1993-01-27
Filing date: 1994-01-23
Publication date: 1996-11-14
Also published as: KR100290497B1; EP0681515B1; DE69409364T2; CN1045556C; RU2108897C1; SE9300232L; DE69409364D1; IL108407A0; ATE164541T1; CN1116834A; KR960700117A; SE9300232D0; EP0681515A1; JPH08505814A; ES2115212T3; WO1994016848A1; US5443334A; SE500703C2

Abstract

A cutting insert comprises an upper side or chip surface, a bottom surface and side faces extending between these two surfaces. At least one side face is provided with a first, possibly helically twisted relief surface and under this first relief surface, a secondary relief surface. Between these two relief surfaces a recess is provided. With this recess, all grinding in the immediate proximity of the cutting edge is avoided. Further, by providing the recess, it is also made possible to precisely position the insert in the milling cutter body, even when the first relief surface is helically twisted.

Description

CUTTING INSERT The present invention relates to a cutting insert for chip forming machining tools, in particular milling cutters, which inserts are produced by form-pressing and sintering of an insert forming powder and compris.es an upper chip surface, a lower plane bottom surface which is intended to be located in abutment with a cooperating bottom support surface of the machining tool, and at least one side surface extending between these surfaces, which side surface can be placed against at least one side abutment surface of the tool and which is generally inclined in an acute angle in relation with the chip surface and in an obtuse angle in relation with the bottom surface, whereby a cutting edge is formed along the transition line between the chip surface and the side surface, adjacent to which cutting edge a clerance surface is provided. Such inserts are more and more produced by means of a direct pressing method at which a cemented carbide forming powder first is conferred the desired form in a pressing tool adapted for the purpose and subsequently is given final strength by sintering in an oven at temperatures above 1000°C.'The pressing operation has been sofisticated over the years and is today so rational that it entails good possibilities of shaping the cutting edge, adjacent chip forming surfaces, possible reinforcing chamfers and clearance surfaces with large exactitude, whereby even the subsequent shrinking at the sintering is included into the computation for the pressing tool dimensioning.

Today's cutting geometries tend towards more and more positive cutting geometries, i.e., larger and larger angles between the cutting insert's chip surface and the normal of the machined surface . The reason for this development is naturally the advantages achieved therewith, such as small cutting forces and a low energy consumption, a well-defined cutting edge for high dimension precision, as well as greater liberty when selecting the clearance angle, with maintained positive cutting geometry. In practice, the limit for the positivity of the chip surface is set by the strength of the cemented carbide, since the cutting edge angle becomes sharper and thereby weaker the more positive the chip surface is.

Positive rake angles also entail that the axial inclination of the cutting insert in the milling cutter body shall be as positive as possible. However, a drawback with increasing inclination angles is that the clearance diminishes with increasing cutting depths . As an example is mentioned that indexable inserts with square basic form and with a side length of 1,5 cm and mounted with 7° positive axial inclination in a milling body of 50 cm diameter in one embodiment has a clearance angle at the cutting insert corner of 10° while corresponding angle at maximal cutting depth is 7°. If the same insert is leant further in the same milling body, to 17 degrees' positive axial angle, the clearance angle at maximal cutting depth diminishes to only 0,7°. This should be compared with the fact that a satisfactory clearance angle should be at least about 7° . This inconvenience is further emphasized at small cutter body diameters.

It is well known for the man skilled in the art that sufficient clearance plays a decisive role for all cutting machining. Inadequate clearance with insufficient free play under the cutting edge results in an accelerated increase of flank wear and vibrations, as well as in chippings and ruptures in the cutting edge. With the aim of guaranteeing sufficient clearance at positive inclination of the inserts in the milling cutter body, a pressing of a helically twisted clearance surface under the cutting edge has been made by modern pressing technique. In this way, one has succeeded in maintaining an essentially constant clearance towards the work piece despite the fact that the insert leans in the milling body. A thereby arisen drawback however is that a combination of a twisted clearance surface next to the cutting edge and a plane secondary clearance surface under the twisted clearance surface results in that the transition or break line between these two clearance surfaces does not become straight and parallel with the cutting edge but curved. As a result of this curved break line, the width of the twisted clearance surface increases towards increasing cutting depth (see Figure 1) . This curved break line between the two clearance surfaces creates problems when determining where the abutment points or surfaces in the cutting insert pocket of the milling cutter body, for axial and radial positioning of the insert, shall be located.

Moreover, for certain types of machining, for instance certain modes of face milling, the requirements on form and dimension precision have become more rigorous during later year. In particular, positive cutting edges require a very high dimension accuracy in order to guarantee a satisfactory function at small tooth feeds. So far, these requirements on accuracy have been accomplished by so called contour grinding, which means that each surface that adjoins the individual cutting edge is after-ground in a step after the sintering. However, a serious drawback of such a contour grinding is that it causes changes in the micro geometry of the cutting insert, i.e., in the surface structure of the cutting edge forming parts of the cutting inserts after a surface treatment, such as blasting, chamfering or deposition of a surface hardening layer, which surface treatment is normally effected as soon as possible after finished sintering. In this manner one may alter the width of occurring negative reinforcing chamfers, the distance from cutting edge to chip forming surfaces, as well as the clearance surface. For instance, a relief surface with an originally pressed, helically twisted shape will wholly or partly be ground away. In practice this implies that the chip forming ability and chip forming function of the insert deteriorates and that its strength and tool life are reduced.

Hence, a first object of the present invention is to avoid each form of grinding in the immediate proximity of the cutting edges.

Another object of the present invention is to enable an exact positioning of the axial and radial abutment points of a cutting insert in an insert pocket, in spite of the provision of a clearance surface which is not ground.

For a preferred embodiment, another object of the present invention is to enable an exact positioning of the axial and radial abutment points of the cutting insert in an insert pocket, in spite of the provision of a helically twisted relief or clearance surface.

According to the present invention, these objects are realized by the features defined in the characterizing part of claim 1, thanks to which one has succeeded in avoiding any form of grinding in the immediate proximity of* the cutting edges. Moreover, by providing a recess in the side surface (s) in accordance with claim 1, the width of the clearance surface becomes essentially constant, even when it is helically twisted. Furthermore, the clearance surface is not influenced in any way at the grinding of the underlying plane side surface, which therefore can be shaped in any desirous manner in order to enable high dimension accuracy at the axial and radial positioning of the cutting insert. An additional advantage is that a large degree of liberty is made possible when selecting different clearances on the plane side surface without influencing the abutment height. Besides, also the advantage is attained that the non-active cutting edges and their adjacent clearance surfaces do not press against the side support surfaces in the insert pocket, wherefore no risk of damages of these in their non- active position exists.

According to the invention the clearance surface adjoining under the cutting edge, also called the first clearance surface, is preferably helically twisted, since this gives the further advantage of retaining the clearance substantially constant, despite the axial positive inclination of the cutting insert in the milling cutter body. However, it can also be wholly plane and have a clearance angle of for example 5 - 25°, preferably 5 - 15°.

For illustrating but non-limiting purposes the invention will now be described in more detail in connection to the appended drawings which show a preferred embodiment.

Figure 1 shows how an insert with helically twistedj clearance or relief surface would appear if it was not provided with a recess according to the present invention .

Figure 2 shows a perspective view of the insert according to the invention from above.

Figure 3 shows a top view of the the insert according to the invention.

Figure 4 shows a cross-section of the the insert along the plane I -I in figure 3.

Figure 5 shows a cutting insert pocket with a transparent insert according to the invention.

Thus, Fig. 1 illustrates how an insert 1' would look like if the clearance surface 3' were helically twisted with increasing clearance in the direction of increasing cutting depth, and being adjacent the plane, secondary clearance surface 2' .

Because of the twisted configuration of surface 3' a curved break line 4' is formed between the two clearance surfaces. The curvature of break line 4' varies considerably with the grinding and the clearance angle of the surface 2', which causes the above mentioned difficulties at the determination of where the abutment points in axial and radial direction in the insert pocket shall be placed.

Figures 2 to 4 illustrate an indexable cutting insert 1 according to the present invention, with a square basic form. The insert comprises an upper side or chip surface 10 and a plane under side or bottom surface 12 which is substantially plane-parallel with the chip surface 10. In this square basic form, four similar side faces extend between chip surface 10 and underside 12. In accordance with the invention, the side faces comprise a first relief surface 3, which preferably is helically twisted in such a manner that the clearance increases with increasing cutting depth. For example, the clearance angle nearest the operational cutting corner can be between 0 and 30° , preferably between 0 and 10°, and at maximal cutting depth between 10 and 40° , preferably between 10 and 20°. At negative axial inclination of the the insert, the clearance surface 3 may be twisted in the other direction, whereby the degree of twisting is of the same order of magnitude as just mentioned above.

Further, the side faces comprise a secondary relief surface 2 and a recess situated between these two relief surfaces, which recess extends along the entire side surface length, from one cutting corner to the next cutting corner. The recess is defined by the surface 4, which is substantially perpendicular to the upper and under sides of the insert, and by the surface 14 which is substantially parallel with said two surfaces. The width B of the clearance surface 3 constitutes suitably 8 to 20 % of the total thickness of the cutting insert. In practice, its smallest width is determined by the strength of the used cemented carbide. The width of the plane surface 4 constitutes suitably between 30 and 50 % of of the total thickness of the cutting insert. Decisive is that the width of the surface 4 and the relief angle of the secondary clearance surface 2 together concede a certain grinding allowance A on the surface 2, without encroaching on the clearance surface 3.

With the aim of reinforcing the protruding part defined by the surfaces 3, 10 and 14, the recess 13 can be provided with a suitable rounding or fillet along the break line between surfaces 4 and 14.

Naturally, the grinding operation is resource demanding in both time and energy. In order to reduce time and energy consumption to a modicum, the insert can be formed with a preferably central recess 5 in connection with the form pressing in each side surface. This recess divides the ground surface 2 on each side surface of the insert into two part surfaces. In practice, the length of recess 5 can be allowed to amount to 25-35% of the whole length of the ground surface 12.

Thus, the recess 13 enables a grinding of the side surfaces 2 which function as abutment surfaces, while the surfaces 3 and 8 adjacent the main cutting edge 6 and the secondary cutting edge 7 are maintained intact in the condition they got after form pressing and sintering. This makes possible the attainment of very high dimension accuracy. Hence, the tolerances of the distance measure between the abutment points of the insert pocket and a diametrally opposite cutting edge can be within the range of 1-20, suitably 1-10 μκι. This is best illustrated in fig 5, in which can be seen the two abutment points or abutment surfaces 16 and 17 of the one support surface and the abutment surface 18 of the other side support surface. The bottom support surface has reference sign 15. Due to the fact that the curved break line 4' in fig 1 thus does not arise, the support surfaces 16, 17 and 16 can be utilized in their whole extent for abutment against corresponding plane surface parts of the cutting insert side faces 2.

The insert according to the invention is preferrably provided with a centrally situated through-hole 11, for insertion: of appropriate clamping means, such as a screw, a locking pin, etc.

As a matter of course, the invention is not limited to the embodiment described and illustrated on the drawings. Thus, it is also possible to apply the invention on inserts with other polygonal basic forms than specifically square, for example on triangular inserts. It would even be possible to prepare circular inserts with a circumferential recess. Moreover, the chip surface can be varied quite considerably: it can for instance be provided with geometries already known as such, such as a positive chip surface, chip breakers, dimples, ridges, bumpies and the like.

According to the working example the main cutting edge is completely straight. It can however also be slightly curved outwards, in order to compensate for the positive axial inclination and to guarantee a complete planity of the milled surface.

Claims

- 10 - 108,407/2 WHAT IS CLAIMED IS:

1. A cutting insert for chip-forming machining tools, in particular milling cutters, comprising: an upper chip surface; a planar bottom surface which is adapted to be located in abutment with a cooperating support surface of the machining tool; at least one side surface extending between the upper and lower surfaces, at least a portion of said one side surface being adapted to be placed in abutment with at least one cooperating side support surface of the machining tool, said at least one side surface being inclined at an acute angle in relation to the chip surface and at an obtuse angle in relation to the bottom surface; at least one cutting edge being formed along a transition line between the upper chip surface and said at least one side surface; a first, helically twisted, relief surface being provided on said at least one side surface adjacent to the cutting edge; a secondary relief surface being provided below said first relief surface, and a recess extending along substantially the entire length of the side surface defining the cutting edge, between the first relief surface and the secondary relief surface; the plane of the secondary relief surface lying outside both the cutting edge and the plane of the first relief surface, as viewed from a point substantially on a midpoint of the side surface of the cutting insert. - 11 - 108,407/2

2. The insert according to claim 1, wherein the secondary-relief surface is substantially planar.

3. The insert according to claim 1, wherein the secondary relief surface is ground.

4. The insert according to claim 1, wherein the recess is ground after forming the insert.

5. The insert according to claim 4, wherein the recess is displaced from the cutting edge such that grinding of said recess does not affect said cutting edge or said first relief surface.

6. The insert according to claim 1, wherein the recess is located inside a plane of the secondary relief surface.

7. The insert according to claim 1, further comprising a second recess formed in said secondary relief surface.

8. The insert according to claim 8 , wherein the secondary recess extends from the recess to the bottom surface.

9. The insert according to claim 8, wherein the secondary recess extends over 25% to 35% of a length of the bottom surface . - 12 - 108,407/2

10. The insert according to claim 1, wherein the recess includes a first surface which is substantially parallel to the bottom surface and is situated adjacent the first relief surface, and a second surface located between said first surface and the secondary relief surface, the second surface being substantially perpendicular to the bottom surface.

11. The insert according to claim 1, wherein the first relief surface is helically twisted such that the clearance increases with increasing cutting depth.

12. The insert according to claim 1, wherein the planes of the upper and bottom surfaces are parallel and wherein the recess includes a first surface that is generally perpendicular to the upper and bottom surfaces.

13. The insert according to claim 12, wherein the recess includes a second surface whose plane is substantially parallel to the planes of the upper and bottom surfaces.

14. The insert according to claim 1, wherein the insert is produced by press-forming and sintering of an insert-producing powder.

15. The insert according to claim 1, wherein the first relief surface extends over 8% to 20% of the thickness of the insert. - 13 - 108,407/1

16. The insert according to claim 1, wherein the insert has a triangular shape.

17. The insert according to claim 1, wherein the insert has a rhombic shape.

18. The insert according to claim 1, wherein the insert has a rhomboidic shape.

19. The insert according to claim 1, wherein the insert has a rectangular shape.

20. A cutting insert for chip-forming machine tools according to claim 1, substantially as hereinbefore described and with reference to the accompanying drawings. for the Applicant: WOLFF, BREGMAN AND GOLLER